Paul C. Nordine

High temperature corrosion in energy systems

At high temperatures, particularly in response to the unique environments associated with the conversion or combustion of fossil fuels, further fundamental studies of alloy reactions with mixed gaseous oxidants are required. Thermodynamic, phase equilibria and diffusion data are lacking in part, and isotope and tracer studies have not been forthcoming. Corrosive thin films of salts and slags on the hardware of gas turbines, heat exchangers, fuel cells and batteries cause an accelerated “hot corrosion”. Thin film electrochemical studies for simple salts and alloys, and supporting thermodynamic studies (solubilities of solids and gases in salts), are required to understand the corrosion mechanism. The effects of several trace gaseous impurities (particularly chlorine) both on the growth and damaging of protective oxide scales and on the degradation of alloy mechanical properties should be studied. High resolution in situ scanning electron microscopy studies could prove fruitful in clarifying uncertain scale growth mechanisms. New protective coating compositions must be found for specific corrosive environments, and more reliable but less expensive coating methods are required. Factors critical to the adhesion of oxide scales (e.g. α-Al2O3 and Cr2O3) on alloys, and the effects of trace alloying elements or dispersed second phases on scale adherence, deserve further attention. The effect on gas-alloy attack of solid deposits, either reactive or relatively inert, should be examined. Electrochemical studies should be made of alloy corrosion in deep salt melts or slags, where the gaseous environment is remote from the alloy surface. The role of grain boundaries in corrosion product scales as short-circuit transport paths for the outward diffusion of metal and the inward ingress of oxygen, sulfur and carbon needs to be clarified. Erosion-corrosion interactions should be studied, with attempts to define the types of coatings that are most resistant to such conditions. Particularly in solar applications, the role of thermal cycling and cyclic stressing on high temperature scaling (corrosion fatigue) needs to be studied. New methods for the monitoring of the concentrations of corrosive components, particularly sulfur and chlorine, in gaseous and fused salt environments require development. The influences of temperature gradients and heat fluxes on material compatibility, redistribution of chemical components and properties of corrosion product layers need further study. High temperature corrosion-resistant alloys excluding the strategic metals chromium and cobalt need to be developed.

Glass fibres of pure and erbium- or neodymium-doped yttria–alumina compositions

Optical fibres doped with lanthanide or transition-metal elements can serve as in-line lasers and amplifiers for fibre-optic telecommunications systems. In general, most such fibre lasers use conventional silica-glass fibres doped with erbium or neodymium. But silicon dioxide absorbs strongly in the infrared for wavelengths of greater than 4 µm or so, limiting the infrared range over which such lasers can operate. Some other oxide materials do not absorb significantly until longer wavelengths—the absorption coefficient of crystalline silica at 4 µm is equal to that of yttrium oxide at 7.1 µm and of sapphire (a form of alumina) at 5.1 µm, for example. Glass fibres made from these materials would therefore expand the range of fibre lasers into the mid-infrared. But molten oxides that do not contain silica typically have a viscosity too low to support fibre-pulling processes. Here we demonstrate that containerless processing, in which a molten sample is levitated by a flow of inert gas, permits sufficient undercooling of molten yttrium aluminium garnet (YAG:Y3Al5O12) to access a viscosity range conducive to fibre-pulling. The process is particularly effective if the molten material of stoichiometric YAG composition is doped with Nd2O3 in place of Y2O3, or with excess Al2O3; and it should also work with other dopants, because molten oxides are good solvents. Fibres could be drawn from a melt doped with Er2O3 in the presence of excess Al2O3. These fibres have the potential to extend the operating range of oxide glass-fibre lasers.

Aero‐acoustic levitation: A method for containerless liquid‐phase processing at high temperatures

A method for containerless liquid‐phase processing was developed which has practical application in process and property research on virtually any material which is involatile at the melting point. It combines aerodynamic and acoustic forces to support and position the levitated material. The design provides forced convection control of the thermal boundary in the gas surrounding beam‐heated specimens, which stabilizes the acoustic forces and allows acoustic positioning necessary to stabilize the aerodynamic levitation forces on molten materials. Beam heating and melting at very high temperatures was achieved. Experiments were conducted on specimens with diameters in the range 0.25–0.4 cm, of density up to 9 g/cm3, at temperatures up to 2700 K, and in oxygen, air, or argon atmospheres. Unique liquid‐phase processing results included deep undercooling of aluminum oxide, glass formation at exceptionally small cooling rates, complete melting and undercooling of YBa2Cu3Ox superconductor materials, direct form...

Levitation apparatus for structural studies of high temperature liquids using synchrotron radiation

A new levitation apparatus coupled to a synchrotron-derived x-ray source has been developed to study the structure of liquids at temperatures up to 3000 K. The levitation apparatus employs conical nozzle levitation using aerodynamic forces to stably position solid and liquid specimens at high temperatures. A 270 W CO2 laser was used to heat the specimens to desired temperatures. Two optical pyrometers were used to record the specimen temperature, heating curves, and cooling curves. Three video cameras and a video recorder were employed to obtain and record specimen views in all three dimensions. The levitation assembly was supported on a three-axis translation stage to facilitate precise positioning of the specimen in the synchrotron radiation beam. The levitation system was enclosed in a vacuum chamber with Be windows, connections for vacuum and gas flow, ports for pyrometry, video, and pressure measurements. The vacuum system included automatic pressure control and multi-channel gas flow control. A phos...

Laser hearth melt processing of ceramic materials

A new technique for synthesizing small batches of oxide‐based ceramic and glass materials from high purity powders is described. The method uses continuous wave CO2 laser beam heating of material held on a water‐cooled copper hearth. Contamination which would normally result during crucible melting is eliminated. Details of the technique are presented, and its operation and use are illustrated by results obtained in melting experiments with a‐aluminum oxide, Y–Ba–Cu–O superconductor material, and the mixtures, Al2O3–SiO2, Bi2O3–B2O3, Bi2O3–CuO. Specimen masses were 0.05–1.5 g.

Aerodynamic levitation of laser-heated solids in gas jets

Solid spheres were aerodynamically levitated in gas jets and laser heated to temperatures above 2000 K. Stable levitation in a supersonic jet from a 0.081‐cm nozzle was demonstrated with 0.03–0.20 g, 0.24–0.47‐cm‐diam specimens at a height between 0.7–2.0 cm above the nozzle and ambient pressures between 1.1–18 Torr. A model of supersonic jet levitation accurately predicts height vs pressure over the full range of conditions that were investigated. The efficiency with which jet momentum is converted into levitation force decreases with the jet:specimen diameter ratio and jet Reynolds number. The rate of jet spreading with distance from the nozzle deduced from levitation experiments agrees with that measured by pitot tube traverses of the jet. Pitot tube pressure measurements also reveal a transition from laminar to turbulent supersonic jet flow at nozzle Reynolds numbers just above the maximum values at which stable levitation is observed. Laser heating reduces the jet momentum required for levitation at ...

Mueller-matrix ellipsometry using the division-of-amplitude photopolarimeter: a study of depolarization effects

A fully automated Mueller-matrix ellipsometer with a division-of-amplitude photopolarimeter as the polarization-state detector is described. This device achieves Mueller-matrix ellipsometry by measuring the Stokes parameters of reflected light as a function of the fast axis C of a quarter-wave retarder, which, in combination with a fixed linear polarizer, determines the polarization state of incident light. The reflected Stokes parameters were Fourier analyzed to give the 16 elements of the Mueller matrix. We investigated depolarization of polarized light on reflection from rough, heterogeneous, and anisotropic surfaces by obtaining measurements on rolled aluminum and plant leaves. The results demonstrate (1) a variation of degree of polarization of reflected light with the input polarization state, (2) the precision with which the measured matrices describe the depolarization results, (3) effects of surface anisotropy (rolling direction) on depolarization and cross polarization by reflection from aluminum surfaces, and (4) large values and differences in the depolarization effects from conifer and deciduous leaves. Depolarization of light reflected by the aluminum surfaces was most sensitive to the angle between the plane of incidence and the rolling direction when the incident Stokes parameters S(1), S(2), and S(3) were equal.

Chemiluminescent titration of F(g) with Cl2(g) and microwave production of atomic fluorine

Chemiluminescent titration of atomic fluorine with Cl2 in a low pressure (133 Pa) flow apparatus has been investigated to establish the accuracy of this titration method and the efficiency of F atom production in a microwave discharge. Up to 90 % dissociation of F2(0.5–10 % in Ar) is achieved by a 100 W Evenson-type cavity 2450 MHz discharge when the microwave power exceeds 2500 kJ mol–1 F and discharge residence time exceeds 0.7 ms. Heterogeneous F atom loss on the fluorine-passivated Al2O3 discharge tube walls occurs with a recombination probability, γF≈ 2 × 10–4.Heterogeneous Cl atom recombination does not interfere with the chemiluminescent F/Cl2 titration in glass apparatus since, in the presence of fluorine, γCl(pyrex) < 8 × 10–5. However, homogeneous three-body recombination to form ClF and/or Cl2 produces errors in the titration endpoint which exceed 10 % if the product of F atom partial pressure, total pressure and the mean residence time between titrant addition and chemiluminescent intensity measurement exceeds 160 Pa2 s at 300 K.

Non-contact temperature measurement

Abstract Investigations of three methods for non-contact temperature measurement are presented. Ideal gas thermometry was realized by using laser-induced fluorescence to measure the concentration of mercury atoms in a Hg-Ar mixture in the vicinity of hot specimens. Emission polarimetry was investigated by measuring the spatially resolved intensities of polarized light from a hot tungsten sphere. Laser polarimetry was used to measure the optical properties, emissivity, and in combination with optical pyrometry, the temperature of electromagnetically levitated liquid aluminum. The ideal gas method is difficult to carry out on earth. It would be applicable to temperature measurements on any material in the quiescent gas environment that may be achieved in space-based containerless experiments, if a convenient method for more accurate and precise density measurements by laser scattering were available. The precision of temperature measurements based on the ideal gas law was ± 2.6% at 1500 – 2300K. The polarized emission technique has the capability to determine optical properties and/or spectral emissivities of specimens over a wide range of wavelengths with quite simple instruments. Its accurate use requires a high degree of specimen stability, position control, and modelling of the experimental measurements. These requirements are avoided by the laser polarimetric method for emissivity measurements which is well developed, uses quite simple instruments, provides nearly instantaneous results, and has been used to measure optical properties and emissivities for a variety of liquids and solids at high temperatures. Further development of the laser polarimetric method is in progress for non-contact temperature measurements in containerless space-based experiments, by combining polarimetric measurements of emissivity with spectral radiation pyrometry.

Properties of high-temperature melts using levitation

Containerless conditions allow well-controlled investigation of liquids at high temperatures. Levitation methods used for this purpose are reviewed, and their application is illustrated by discussion of the properties and behavior of deeply undercooled yttrium-aluminum-oxide melts.